The present invention relates to a barium sulfate scale inhibitor composition containing a water-soluble carboxylic acid or sulfonate polymer having phosphate functionality. The invention also relates to a method for reducing calcium carbonate and/or barium sulfate scale in high scaling environments, especially in subterranean oil fields. Additionally the scale inhibitor can be detected by inductively coupled plasma-atomic emission spectroscopy (ICP) or UV-vis, providing a method for measuring concentration of inhibitor in both downhole and topside treatments.
Subterranean oil recovery operations can involve the injection of an aqueous solution into the oil formation to help move the oil through the formation and to maintain the pressure in the reservoir as fluids are being removed. The injected water, either surface water (lake or river) or seawater (for operations offshore) contains soluble salts such as sulfates and carbonates. These salts may be incompatible with the ions already contained in the oil-containing reservoir (formation water). The formation water may contain high concentrations of certain ions that are encountered at much lower levels in normal surface water, such as strontium, barium, zinc and calcium. Partially soluble inorganic salts, such as barium sulfate and calcium carbonate, often precipitate from the production water as conditions affecting solubility, such as temperature and pressure, change within the producing well bores and topsides. This is especially prevalent when incompatible waters are encountered such as formation water, seawater, or produced water.
Barium sulfate and strontium sulfate form very hard, very insoluble scales that are difficult to prevent. Barium and strontium sulfates are often co-precipitated with radium sulfate, making the scale mildly radioactive and introduces handling difficulties. Unlike common calcium salts, which have inverse solubility, barium (strontium and radium) sulfate solubility is lowest at low temperature, and this is particularly problematic in processing oil where the temperature of the fluids decreases. Modern extraction techniques often mean that the temperature of the produced fluids (water, oil and gas mixtures/emulsions) are decreased (as low as 5 C) and contained in production tubing for long periods (24 hrs or longer). Calcium carbonate can be readily removed using HCl acid washing should scale occur. This can be performed topside or downhole, is cheap, and is non-invasive. Dissolution of sulfate scales is difficult (requiring high pH, long contact times, heat and circulation) and can only be performed topside. Alternatively, milling and in some cases high-pressure water washing can be used. These are expensive, invasive procedures and require process shutdown. Inhibition is the key approach to sulfate scales, especially downhole.
Barium sulfate, or other inorganic supersaturated salts, can precipitate onto the formation to form a scale, thereby clogging the formation and restricting the recovery of oil from the reservoir. The insoluble salts may also precipitate onto production tubing surfaces and associated extraction equipment that can limit productivity, limit production efficiency, and compromise safety. Certain oil-containing formation waters are known to contain high barium concentrations of 400 ppm, and higher. Since barium sulfate forms a particularly insoluble salt, the solubility of which declines rapidly with temperature, it is difficult to inhibit scale formation and to prevent plugging of the oil formation and topside processes and safety equipment.
While “scale inhibition” and “deposit control” are generic terms without mechanistic implications, there are two generally accepted mechanisms for controlling the amount of divalent metal ions fouling or depositing in the surface of the formation: 1) inhibiting precipitation of the material from the process water, or 2) dispersing the material once it has formed, to prevent it from attaching to the surfaces. The exact mechanism by which a particular scale inhibitor functions, and the interplay between these two or other mechanisms is not well understood. The compositions of the present invention may operate by either or both of these routes.
Current methods for inhibiting barium sulfate scaling involve the use of expensive organic phosphonic acids, as described in U.S. Pat. Nos. 6,063,289 and 6,123,869. Acrylic polymer scale inhibitors containing a phosphino or phosphono moiety are also used. U.S. Pat. No. 4,209,348 describes a copolymer of (meth)acrylic acid having a phosphate functionality that is useful as a combined scale and corrosion inhibitor in industrial water treatment. This chemistry provides only limited adhesion to the oil-containing formation. U.S. Pat. No. 4,711,725 describes the use of terpolymers of (meth)acrylic acid/2-acrylamido-2-methyl propane sulfonic acid/substituted acrylamides for inhibiting the precipitation of calcium phosphate.
EP 459661 A1 describes a method for silica scale inhibition using a mixture of aluminum or magnesium ions with a low molecular weight poly(meth)acrylic acid or polymaleic acid, plus either a copolymer or a terpolymer of a) (meth)acrylic acid or maleic acid with b) (meth)acrylamido methyl propane sulfonic acid, or styrene sulfonic acid, and c) another monomer which could be a vinyl ester, and the vinyl ester could contain a phosphate group.
Surprisingly it has been found that the addition of a phosphate moiety to a polyacrylate or polysulfonate scale inhibitor allows for greater adsorption to an oilfield reservoir, thus allowing for an increase in the treatment lifetime, while still retaining good scale inhibition properties. Polymeric inhibitors also have the advantage of being relatively unmetabolized, and therefore have low toxicity and bioaccumulation characteristics.